ABSTRACT
Structural organization of hydrogen and oxygen functionalized nanodiamond (ND) particles in hydrosols was investigated using a cryo-TEM method. The formation of chain-like structures was observed for hydrogen functionalized NDs while oxygen functionalized NDs tend to form more compact structures. In order to understand possible interaction mechanisms between NDs in hydrosols and to explain these experimental results, first-principles calculations were performed. Charged H-terminated ND particles and particles with partially dissociated hydroxyl and carboxyl groups on the surface were investigated as models of a real ND particle in solution. For positively charged H-terminated particles, it was established that charge distribution is determined by the values of valence band maximum for the particle facets. For negatively charged oxygen functionalized particles, the charge is localized near functional groups. In both cases, interaction is determined by the interplay between Coulomb interaction and van der Waals attraction. For detailed analysis of the ND interaction, the continual model considering this interplay was developed. The results obtained with this model indicate that the formation of chain structures from linked ND particles is caused by charge separation inside the ND particle. For the H-terminated ND particles in water solution, strongly anisotropic distribution of electrostatic potential around the particles promotes formation of non-compact chain structures of particles via interaction between facets with opposite charges. This effect of charge separation is lower in the oxygen functionalized particles as the charge is localized at the uniformly distributed O-containing functional groups, thus, more compact structures can be formed. These general qualitative statements address the problem of interactions between the large number of ND particles and explain the presented cryo-TEM experimental results.
ABSTRACT
The effect of the hydrophobic block length in diblock (PLLA x- b-PEO113, x = 64, 166, 418) and triblock (PLLA y- b-PEO91- b-PLLA y, y = 30, 52, 120) copolymers of l-lactic acid and ethylene oxide on the structure of micelles prepared by dialysis was studied by wide- and small-angle X-ray scattering in dilute aqueous solution, dynamic light scattering, transmission electron microscopy, atomic force microscopy, and force spectroscopy. It was found that the size of the crystalline PLLA core is weakly dependent on the PLLA block length. In addition to individual micelles, a number of their micellar clusters were detected with characteristic distance between adjacent micelle cores decreasing with an increase in PLLA block length. This effect was explained by the change in the conformation of PEO chains forming the micellar corona because of their overcrowding. Force spectroscopy experiments also reveal a more stretched conformation of the PEO chains for the block copolymers with a shorter PLLA block. A model describing the structure of the individual micelles and their clusters was proposed.
